I think you're overstating the defensive importance of that winger with Bergeron and Marchand. I saw you mention that CJ won't play someone there if they're not good defensively and history has shown that not to be the case.
Seguin was terrible in his own zone. Jagr was worse (at least Seguin back checked). Reilly Smith played there in his first full season in the league, when he was still learning CJ's system and NHL defense in general. There's nothing noteworthy about Connolly's defensive game. Or even Mark Recchi for that matter, though he was probably the best of the bunch (and that's not saying muck). Eriksson, who is great defensively, wasn't really used there at all.
Bergeron and Marchand are better 4on5 than most lines 5on5. I think that spot on their RW is a freebie. Anyone can play there. Big/small, fast/slow, old/young, skilled/grinder. They just had their best years with (statistically) the worst RW they've every played with. Give that spot to whichever kid looks the most ready (Heinen) and let him run with the bulls.
The difference back then it wasn't a big of a concern as the D was much stronger defensively. Match-ups weren't nowhere near as big of an issue. Julien began to move away a bit last season from rolling 4 lines and using his D for match-ups as often. He realized he doesn't really have the personnel on the back-end to run his D the way he used to.
And they don't have to be extremely strong defensively, just trusted by their coach. There is a difference.
Hence no Spooner or Pastrnak. Seguin by year 2 had some trust there, as did Smith (who was underrated defensively). Connolly had it, as did Stempniak and Recchi.
Jagr I can't comment because he was there more because Seguin was struggling and Julien's options were limited (neither belonged on the 4th line, and he didn't want to bust up Lucic-Krejci-Horton).
Go look at his use last year of Connolly and Stempniak. 82 games and we never saw Pastrnak there, minimal time for Spooner, defensively weak Hayes was rarely used there either. There is a reason for that.
I have no problem with giving Heinen that spot if he's ready, regardless of his defensive game. But I'm not Julien, and that's how Julien is going to roll if he doesn't trust Heinen defensively.
Anyways kid has to make the team first. Like many here I like his chances but I've been fooled before.